Conventional nitrogenous thermosetting crosslinking resins, or aminoplasts, are widely used in the coatings industry. Urea, melamine, substituted guanamine, and related triazines have each been the subject of much effort in the search for improved crosslinking resins. Such resins are formed by reacting the selected amine or amide with formaldehyde to form methylol groups which may or may not be temporarily blocked against excessive condensation by etherification with one of the lower alcohols. As a class, these resins are characterized by their ability to form hard, durable films under the influence of heat and/or suitable catalysts and are considered to be relatively rapid-curing.
It is often necessary, when formulating aminoplast crosslinking resins, to sacrifice one desired property in order to attain or enhance another. For example, melamine-formaldehyde resins are noted for their excellent color retention, hardness, and resistance to hydrolytic, caustic, detergent, or salt spray breakdown. Although benzoguanamine-formaldehyde resins surpass the melamine resins in imparting flexibility and chemical resistance, they are often severely deficient in exterior durability. Urea-formaldehyde resins promote superior intercoat adhesion, but their color and gloss retention are poorer than that of melamine resins.
As the search for new and improved aminoplast resins continues, emphasis will be placed on the overall balance of properties possessed by these resins, as well as on commercial availability and cost.